A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An internal gaseous environment in the region of the patterning device may be controlled to prevent contaminants and/or gas (e.g., air) with a variable property from interfering with the radiation beam and/or a sensitive element of the patterning device. The internal gaseous environment will typically be substantially isolated from the region outside, but not perfectly sealed off. A gas supply system having an outlet into the internal gaseous environment may be provided and configured to maintain an overpressure in the internal gaseous environment. The overpressure may drive a constant flow of gas out of the internal gaseous environment. The constant outward flow of gas may help to prevent inflow of contaminants. The constant flow of gas may be channeled through a leaky seal, for example through opposed flow-restricting surfaces.
A support for the patterning device may predominantly be configured to be driven linearly in a certain direction (and thus predominantly drive the patterning device in the direction). The direction may be perpendicular to the axis of the projection system, which is referred to as the Z-axis. The direction may be referred to as the Y-direction.